Oxides of Nitrogen:  Oxides are binary compounds that have one or more oxygen atoms as well as another element in their composition. Nitrogen gas is the most abundant gas in the atmosphere and a major contributor to life on Earth. The element nitrogen belongs to Group \(15\) of the periodic table. It is a nonmetal that exists in the gaseous state. Gaseous nitrogen accounts for approximately \(78\% \) of the volume of our atmosphere. Nitrogen reacts with oxygen to form a variety of oxides. The oxides of nitrogen are very useful in various fields- like as an anaesthetic, as a fuel and in the synthesis of nitric acid. In this article, we are going to discuss the different oxides of nitrogen in detail and their uses.

Oxides of Nitrogen: Overview

Nitrogen combines with oxygen under various conditions to form a number of binary oxides that differ depending on the oxidation state of the nitrogen atom. The oxidation number can range between \( + 1\) and\(+ 5.\) The tendency of oxides to form pπ-pπ multiple bonds determines their structure.

Below given is the table with names of nitrogen oxides along with their formula and oxidation state:

Oxides of Nitrogen

Name	Formula	Oxidation State of Nitrogen
Nitrogen (I) oxide/Nitrous oxide	\({{\text{N}}_2}{\text{O}}\)	\( + 1\)
Nitrogen (II) oxide/Nitric oxide	\({\text{NO}}\)	\( + 2\)
Nitrogen (III) oxide/Dinitrogen trioxide	\({{\text{N}}_2}{{\text{O}}_3}\)	\( + 3\)
Nitrogen (III) oxide/Nitrogen dioxide/Dinitrogen tetroxide	\({\text{N}}{{\text{O}}_2}/{{\text{N}}_2}{{\text{O}}_4}\)	\( + 4\)
Nitrogen (III) oxide/Dinitrogen pentoxide	\({{\text{N}}_2}{{\text{O}}_5}\)	\( + 5\)

Sources of Nitrogen Oxides

According to scientists, nature produces between \(20\) and \(90\) million tonnes of nitrogen oxides per year on Earth. Volcanoes, oceans, biological decay, and lightning strikes are all-natural sources. Every year humans add another \(24\) million tonnes of nitrogen oxides to the atmosphere.

When oxygen and nitrogen combine at high temperatures in the atmosphere, \({\rm{NO}}\) and \({\rm{N}}{{\rm{O}}_2}\) are produced. Automobile and truck exhaust gases and emissions from power plants are major sources of nitrogen oxides. Although automobile exhaust contains more \({\rm{NO}}\) and \({\rm{N}}{{\rm{O}}_2}\) once released into the atmosphere, the \({\rm{NO}}\) quickly combines with oxygen in the air to form \({\rm{N}}{{\rm{O}}_2}\) Let us study some of the oxides of nitrogen:

Nitrous Oxide or Dinitrogen Oxide \({{\rm{N}}_2}{\rm{O}}\)

Nitrous oxide is called laughing gas because it causes hysterical laughter when inhaled. It is usually prepared by gently heating ammonium nitrate to \({\rm{240}}\) degrees Celsius.

\({\text{N}}{{\text{H}}_4}{\text{N}}{{\text{O}}_3}\xrightarrow{{{\text{heated}}\,{\text{at}}\,{{240}^{\text{o}}}{\text{C}}}}{{\text{N}}_2}{\text{O}} + 2{{\text{H}}_2}{\text{O}}\)

It is a colourless gas that is somewhat soluble in cold water but not at all soluble in hot water. Nitrous oxide is a non-reactive oxide.

Structure

Nitrous oxide has a linear structure that is a resonance hybrid of the two forms. The bond lengths in the atom are \(113\) and \(119\) per molecule, respectively.

Nitrogen Monoxide or Nitric Oxide \({\rm{NO}}\)

Nitrogen monoxide can be produced by reacting nitrite salt with ferrous sulphate.

\(\underset{{{\text{Sodium}}\,{\text{nitrite}}}}{\mathop {{\text{2NaN}}{{\text{O}}_{\text{2}}}}} {\text{ + }}\underset{{{\text{Ferrous}}\,{\text{sulphate}}}}{\mathop {{\text{2FeS}}{{\text{O}}_{\text{4}}}}} {\text{ + }}\underset{{{\text{Sulphuric}}\,{\text{acid}}}}{\mathop {{\text{3}}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}}}  \to \)
\(\underset{{{\text{Ferric}}\,{\text{sulphate}}}}{\mathop {{\text{F}}{{\text{e}}_{\text{2}}}{{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)}_{\text{3}}}}} {\text{ + }}\underset{{{\text{Sodium}}\,{\text{hydrogen}}\,{\text{sulphate}}}}{\mathop {{\text{2NaHS}}{{\text{O}}_{\text{4}}}}} {\text{ + }}\underset{{{\text{Water}}}}{\mathop {{\text{2}}{{\text{H}}_{\text{2}}}{\text{O}}}} {\text{ + }}\underset{{{\text{Nitric}}\,{\text{oxide}}}}{\mathop {{\text{2NO}}}} \)

The catalytic oxidation of ammonia can also prepare it at \(1100\,{\rm{K}}\) in the presence of platinum.

\({\text{4N}}{{\text{H}}_3} + 5{{\text{O}}_2}\xrightarrow{{1100\,{\text{K}},{\text{Pt}}}}{\text{4NO}} + 6{{\text{H}}_2}{\text{O}}\)

It can also be prepared by the reaction of nitric acid on copper.

\({\rm{3Cu}}\,{\rm{ + }}\,{\rm{8HN}}{{\rm{O}}_3} \to 3{\rm{Cu(N}}{{\rm{O}}_3}{)_2} + 2{\rm{NO}}\,{\rm{ + 4}}{{\rm{H}}_2}{\rm{O}}\)

Nitric oxide is a colourless gas that, when melted, takes on a blue colour. It is a very weak solvent in water. It is also a neutral oxide and is also paramagnetic in gaseous form and diamagnetic in solid and liquid form.

Nitric oxide forms complexes with transition metals, which causes the brown ring test for nitrates. At higher temperatures, it decomposes into elements due to thermodynamic instability.

Structure

It is a molecule with a linear shape and is a resonance hybrid of the two structures. The bond length is \({\rm{115}}\) pm which is intermediate between the double bond and triple bond.

Dinitrogen Trioxide \({\rm{(}}{{\rm{N}}_2}{{\rm{O}}_3})\)

When a mixture of nitric oxide and nitrogen dioxide or dinitrogen tetraoxide is cooled to below minus \(20\) degrees Celsius, it forms a blue liquid called dinitrogen trioxide.

\(\mathop {2{\text{NO}}}\limits_{{\text{Nitric}}\,{\text{Oxide}}} + \mathop {{{\text{N}}_2}{{\text{O}}_4}}\limits_{{\text{Dinitrogen}}\,{\text{tetroxide}}} \xrightarrow[{{\text{Below}}\, – {{20}^{\text{o}}}{\text{C}}}]{{{\text{Cooling}}}}\mathop {2{{\text{N}}_2}{{\text{O}}_3}}\limits_{{\text{Dinitrogen}}\,{\text{trioxide}}} \)

Dinitrogen trioxide is a nitrous acid anhydride that is acidic in nature. It exists only as a solid at very low temperatures and dissociates into \({\rm{NO}}\) and \({\rm{N}}{{\rm{O}}_2}\) above its melting point.

Structure

It is a resonance hybrid of the two forms with a planar shape, as shown below.

Nitrogen Dioxide ( \({\rm{N}}{{\rm{O}}_2}\))

Nitrogen Dioxide can be made by heating lead nitrate to approximately \({\rm{673}}\,{\rm{K}}\)

\(\mathop {2{\text{Pb}}{{\left( {{\text{N}}{{\text{O}}_3}} \right)}_2}}\limits_{{\text{Lead}}\,{\text{nitrate}}} \xrightarrow{{{\text{heat}},\,673\,{\text{K}}}}\mathop {4{\text{N}}{{\text{O}}_2}}\limits_{{\text{nitrogen}}\,{\text{dioxide}}} + \mathop {2{\text{PbO}}}\limits_{{\text{lead}}\,{\text{oxide}}} + \mathop {{{\text{O}}_2}}\limits_{{\text{oxygen}}} \)

Nitrogen Dioxide is a brown gas that is a combined anhydride of nitrous acid and nitric acid. It’s a type of acidic oxide. The atom is a paramagnetic substance that exists as a resonance hybrid. Due to the close proximity of an unpaired electron, it dimerises to a colourless dinitrogen tetroxide atom with an even number of electrons. Nitrogen dioxide binds to solid dinitrogen tetroxide at low temperatures.

Structure

It is angular in shape, with a bond angle of \(134\) degrees and a bond length of \(120\,{\rm{pm}}\).

Dinitrogen Pentoxide \({\rm{(}}{{\rm{N}}_2}{{\rm{O}}_5})\)

Dinitrogen pentoxide is produced by dehydrating nitric acid with phosphorus pentoxide.

\(\mathop {4{\text{HN}}{{\text{O}}_3}}\limits_{{\text{Lead}}\,{\text{nitrate}}} + \mathop {{{\text{P}}_4}{{\text{O}}_{10}}}\limits_{{\text{Phosphorous}}\,{\text{pentoxide}}} \xrightarrow{{{\text{Dehydration}}}}\mathop {4{\text{HP}}{{\text{O}}_3}}\limits_{{\text{Metaphosphoric}}\,{\text{acid}}} + \mathop {2{{\text{N}}_2}{{\text{O}}_5}}\limits_{{\text{Dinitrogen}}\,{\text{pentoxide}}} \)

It is a colourless crystalline deliquescent solid that dissolves easily in water to form nitric acid. It is a nitric acid anhydride that is acidic in nature.

Structure

It is a resonance hybrid of the two forms with a planar shape, as shown below. The planar structure is only visible in its vapour state; it is ionic in its solid-state. Nitronium nitrate is the name given to the ionic solid.

Nitrogen Oxides as Pollutants

Several types of air pollution are caused, at least in part, by nitrogen oxides. The reddish-brown haze we call smog is coloured by nitrogen dioxide. The photodissociation of nitrogen dioxide by sunlight creates nitric oxide and ozone in the troposphere, which is another component of smog. A series of chemical reactions convert Volatile Organic Compounds (VOCs) into substances that react with nitrogen dioxide to form PAN (Peroxyacytyl nitrate), yet another component of smog. Nitrogen dioxide in the air also reacts with water vapour to form nitric acid, which is one of the acids in acid rain.

The concentration of nitrogen dioxide in clean air is around \({\rm{10}}\) parts per billion \({\rm{(ppb)}}\). In smog, the concentration more than doubles to around \({\rm{10}}\) \({\rm{(ppb)}}\).

Uses

1. Nitrous oxide (laughing gas) is a safe anaesthetic used in dentistry, ambulances, and childbirth and is valued for its anti-anxiety properties.
2. Nitric oxide is breathed in. It works by relaxing the smooth muscle to widen (dilate) blood vessels, particularly those in the lungs. In premature babies, nitric oxide is combined with a breathing machine (ventilator) to treat respiratory failure.
3. Because of its high combustibility, dinitrogen trioxide is used as a special purpose fuel. The chemical only aids combustion and does not itself burn. It is more commonly used as an oxidizer in conjunction with other chemical compounds.
4. Nitrogen dioxide is used as a byproduct in the synthesis of nitric acid. It is used to produce oxidised cellulose compounds, as a catalyst, as an intermediate in the synthesis of sulphuric acid. It is also used as an oxidizer in rocket fuels, as a nitrating agent, as an oxidising agent, and used in the manufacture of explosives.
5. Nitrogen pentoxide is used in high-fuel rockets, and it is used as a strong oxidizer in non-water-based solvents to easily nitrate molecules that are extremely sensitive to water. In modern synthetic organic chemistry, it is used as a nitrating agent.

Summary

We can conclude that nitrogen, under various conditions, combine with oxygen and form its oxides. We studied that these oxides have an oxidation number range between \({\rm{ + }}\) and \({\rm{ + 5}}\). Now we know about the five oxides of nitrogen- nitrous oxide, nitric oxide, dinitrogen trioxide, nitrogen dioxide, and dinitrogen pentoxide. We also studied some of the important uses of all these oxides – they are used as anaesthetics, as a byproduct in the synthesis of nitric acid, and in high-fuel rockets.

The structures and the preparation and properties of the oxides of nitrogen can be summarised as-

FAQs on Oxides of Nitrogen

Q.1. What are the properties of oxides of nitrogen?
Ans:  Some of the properties of oxides of nitrogen are as follows-
i. Dinitrogen oxide is a colourless, non-flammable gas with neutral properties. It is commonly referred to as laughing gas.
ii. Dinitrogen trioxide is a dark blue solid with a strong acidic odour. It can be isolated only at low temperatures, i.e., in the liquid and solid phases. The equilibrium favours the formation of constituent gases as the temperature rises.
iii. Nitrogen dioxide is a reddish-brown toxic gas with a distinctive sharp, biting odour, a major air pollutant. It has an acidic pH
iv. Dinitrogen pentoxide is a colourless solid that sublimes at slightly higher temperatures than room temperature. It is a volatile and potentially hazardous oxidizer.
v. Nitrogen Dioxide is a brown gas that is a combined anhydride of nitrous acid and nitric acid. It’s a type of acidic oxide.

Q.2. How is nitrogen oxide prepared?
Ans: Nitrogen oxides are produced during combustion by the reaction of nitrogen and oxygen gases in the air, particularly at high temperatures. Oxygen and nitrogen gases do not react together at room temperature. A lightning strike naturally produces nitrogen oxides.
Nitrogen oxides are produced in large cities as a result of fuel combustion in mobile and stationary sources.

Q.3. What is the structure of nitrogen dioxide?
Ans: The formula of nitrogen dioxide is \({\rm{N}}{{\rm{O}}_{\rm{2}}}\) Its structure is-

Q.4. What are the five oxides of nitrogen?
Ans: Oxides of nitrogen are a gas mixture made up of nitrogen and oxygen. The five oxides of nitrogen are as follows-
i. Nitric oxide \({\rm{NO}}\)
ii. Nitrogen dioxide \({\rm{N}}{{\rm{O}}_{\rm{2}}}\)
iii. Dinitrogen oxide \({{\rm{N}}_2}{\rm{O}}\)
iv. Dinitrogen trioxide \({{\rm{N}}_2}{{\rm{O}}_3}\)
v. Dinitrogen pentoxide \({{\rm{N}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}}\)

Q.5. Do oxides of nitrogen impact the environment?
Ans: High levels of oxides of nitrogen can harm vegetation by making it more vulnerable to disease and frost damage. Ozone is formed when oxides of nitrogen react with other pollutants in the presence of sunlight. Ozone in high concentrations is also harmful to vegetation.

Q.6. What is the chemical formula for nitrogen oxide?
Ans: The chemical formula of nitrogen oxide is \({\rm{N}}{{\rm{O}}_{\rm{x}}}\)

Learn About Oxides Of Sulphur Here

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